Methane Hydrate Formation and Dissociation in Oil-in-Water Emulsion

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Methane Hydrate Formation and Dissociation in Oil-in-Water Emulsion Himangshu Kakati, Shranish Kar, Ajay Mandal, and SUKUMAR LAIK Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef500681z • Publication Date (Web): 02 Jul 2014 Downloaded from http://pubs.acs.org on July 6, 2014

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Methane Hydrate Formation and Dissociation in Oil-in-Water Emulsion Himangshu Kakati, Shranish Kar, Ajay Mandal* and Sukumar Laik Gas Hydrate Laboratory, Department of Petroleum Engineering Indian School of Mines, Dhanbad, India-826004. *

Corresponding Author: E-Mail: [email protected], Fax: 91-326-2296632

ABSTRACT: The formation and dissociation of methane hydrates in oil in water emulsions have been studied. The phase equilibrium of methane hydrate in emulsion has been investigated and enthalpy of dissociation of methane hydrates in emulsion has been calculated using Clausius-Clapeyron equation based on the measured phase equilibrium data. The kinetics of hydrate formation has also been studied to observe the induction time of hydrate formation and hydrates formation rate. Further the amount of gas consumed during hydrate formation has been calculated using the real gas equation. Keywords: Gas hydrate, Formation, Dissociation, Emulsion, Enthalpy 1.

INTRODUCTION Gas hydrates are naturally occurring non-stoichiometric crystalline compounds composed

of water and gas, where gas molecules are trapped within the cavities formed by water molecules with the help of hydrogen bonding.1 Gas hydrates form when water and hydrate forming gas comes in contact under high pressure and low temperature conditions.2 There are three known common hydrate structures (Structure I, II and H) depending principally on the molecular size of the guest molecule.1 In hydrocarbon industry emulsions can be encountered in almost all phases of oil production inside the reservoirs, well bores and well heads, transportation through pipelines, during crude storage and petroleum processing. Within the reservoir when crude oil remain associated with formation water, emulsions are often formed with mixing and/or changes in pressure, temperature and presence of asphaltene, resin, fine solid particle etc.3 Again when such emulsions are associated with lighter hydrocarbon gases, there is a greater possibility of forming hydrates under the condition of reservoir pressure and temperature. As such water is invariably produced with crude oil and natural gas. In the flow lines under high pressure and

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enough shear forces crude oil flows with water, as a result stable oil-water emulsion may form. In this case also under favourable temperature and pressure the associated natural gas may form hydrates. Formation of hydrates in subsea pipeline is a major concern for multiphase transportation. Low temperature and high pressure in offshore pipelines provides a perfect environment for hydrates formation.1,4 Upon formation, hydrates may either flocculate and form dispersion, which is transported with the fluid or agglomerate and form plug leading to flow line blockage.5-7 A detailed Knowledge of kinetics of hydrate formation, accumulation, and decomposition processes are key factors during selection of inhibitors and creation of new technologies for flow assurance to prevent hydrate problems.8 Furthermore, the formation of hydrates in drilling fluids, especially oil base drilling fluids and its inhibition is the object of numerous investigations, which requires the knowledge of hydrates formation and dissociation.9-11 Several researchers had studied the kinetics of hydrates formation and dissociation in emulsion. Greaves et al.7 studied methane hydrate formation and dissociation in emulsions at high water cuts. They observed that hydrate formation and dissociation from water-in-oil (W/O) emulsions destabilized the emulsion, with the final emulsion formulation favoring a water continuous state following re-emulsification. Hence, following dissociation, the W/O emulsion formed a multiple O/W/O emulsion (60 vol% water) or inverted at even higher water cuts, forming an oil-in-water (O/W) emulsion (68 vol% water). In contrast, hydrate formation and dissociation from O/W emulsions (>71 vol% water) stabilized the O/W emulsion. Hoiland et al.5-6 had studied hydrates in emulsions to see whether presence of hydrate particles can promote or delay the inversion of a water-crude oil emulsion (i.e, O/W to W/O or vice-versa). Sinquin et al.12 studied kinetic of hydrate formation in presence of four different crudes (one condensate, one low asphaltenic content crude, one high asphaltenic content and another one is paraffinic crude). They found that there was rapid and violent formation of hydrates for condensate and paraffinic crude. On the otherhand for asphaltenic crude (both high and low) hydrates formation is slow and smooth. Talatori et al.8 studied the formation of mixed gas hydrates of methane, ethane and propane in crude oil emulsions with different water cuts in a stirred constant volume high pressure cell. Xiang et al.13 studied hydrate formation and dissociation of natural gas in (diesel oil + water) emulsion 2


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systems for five water cuts: 5, 10, 15, 20, and 25 vol%. They found that the dissociation rate and the amount of dissociated gas increase with the increase in water cut. Dalmazzone et al. 14

studied the methane hydrate dissociation equilibrium in water-in-oil emulsions using

differential scanning calorimetry and found that hydrate formation in emulsions is related to the magnitude of the subcooling. Irvin et al.15 studied the mechanism of hydrate formation in water-in-oil

microemulsions.

Lachance

et

al.16

studied

the

effect

of

hydrate

formation/dissociation on emulsion stability using DSC and visual techniques. They found that hydrate formation/dissociation is effective at destabilizing emulsions. But the asphaltene fraction of crude oils resists hydrate-induced destabilization. In this paper we have studied the formation and dissociation of methane hydrates in emulsions with different oil content. The work is focused on how formation and dissociation behavior changes with increasing the oil percentage in the emulsion. The kinetics of hydrate formation has also been studied to observe the induction time of hydrate formation which is a very important parameter of hydrate formation. Gas consumption and formation rate during hydrates formation has also been studied.

2.

EXPERIMENTAL SECTION

2.1 Apparatus. The schematic diagram of the gas hydrate autoclave apparatus is shown in Figure 1. The high-pressure hydrate autoclave procured from Vinci Technology, France, was used to study gas hydrate formation and dissociation. The apparatus measures the induction time of hydrate formation and pressure as a function of time during hydrate formation, and takes video graphs during the experiments. The system consists of a constant volume hydrate cell with a capacity of 250 cm3 and pressure rating up to 20.68 MPa. The cell is a stainless steel cylinder where a stethoscopic camera, a thermocouple and a pressure digital gauge is fitted to the top of the cell. The cell temperature is controlled by a thermostatic bath. The bath works with a thermostatic fluid (a mixture of 85% water and 15% glycol) with operating temperature range -10°C to 60°C. The bath size is about 225×370×429 mm. The thermocouple measures the temperature inside the cell with an accuracy of 0.1oC. The cell 3


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pressure is monitored by a pressure transducer. A magnetic stirrer with adjustable rotation speed (up to 1000RPM) is used to agitate the test fluid. Autoclave set up is also attached with gas booster to build up high pressure inside hydrate cell and a vacuum pump is used to evacuate air in the cell before inserting gas.

2.2 Materials. All the experiments were performed with 99.99% pure methane (collected from Chemtron Science Laboratory, Navi Mumbai, India) and reverse osmosis water from Millipore water system (Millipore SA, 67120 Molshein, France). Crude oil used for preparation of emulsion was collected from Ahmedabad oil field (India). The physico-chemical characteristics of crude oil are given in the Table 1. SARA distribution (Saturates, Aromatics, Resins and Asphaltenes) was determined using liquid column chromatography through elution using different solvents of varying polarity. In the first stage, asphaltenes and insoluble resins are separated by precipitation with n-hexane. The mixture is cooled at 30 °C and precipitated asphaltenes are filtered out. The filtered sample (maltene fraction) is later split in a chromatographic column to obtain saturated compounds, aromatic, and polar resins. The solid asphaltenes were washed with n-heptane before drying and then weighed. The chromatographic separation of maltenes is carried out in an installation that comprises glass columns packed with silica gel. The different fractions are separated depending on their affinity to the solvent being used at each step of extraction. Trichloro-methane is used to recover resins, n-hexane is used for saturates and hot toluene is used to extract aromatics. The solvents used are extracted using Soxhlet apparatus and percentage of each fraction (weight % of crude oil) is calculated. The SARA analysis of the used crude oil is reported in Table 2. 2.3 Procedure. For preparation of synthetic oil-in-water emulsion crude oil is used as dispersed phase and distilled water is used as continuous phase and no surfactant is added to making emulsion because natural surfactants like asphaltenes, waxes, resins and naphthenic acids etc. are present in crude oil which stabilizes the emulsion. Measured amount of crude oil and distilled water mixture was taken in a flask and stirred at 2500RPM for 5hours using an mechanical stirrer (Remi RQ-20 Plus). Then the emulsion was kept for settling in a 4


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separating funnel for removing the undissolved crude.

After separating the emulsion from

separating funnel its concentration in ppm was measured in Infracal TOG/TPH analyzer (Wilks) to know the actual oil content in emulsion.

The hydrate formation may occur in the

reservoir in presence of formation water or in the pipeline during production and transportation of oil and gas in presence of produced water.The concentration of oil in formation water or produced water varies widely ranging between 100 and 1000 mg/L or still higher depending on composition of crude oil17-18. Based on literature review three different emulsion with oil concentration 870 ppm, 1880 ppm and 3560 ppm are used in the present study. For hydrate formation dissociation study, the autoclave cell was filled with 120 cm3 of emulsion and immersed into a temperature controlled bath. Before inserting the methane gas, the air in the cell was removed by a vacuum pump. The cell was then pressurized with methane gas up to the desired pressure. The temperature of the cell at the time of charging was 20°C. After charging the gas, the system was kept at that temperature for one hour. The cell was then cooled down stepwise (18°C, 16°C, 14°C etc.) in the programmable bath to attain the hydrate formation environment. The system was kept at each temperature for one hour to attain the equilibrium condition. Before hydrate formation the oil-water emulsion is saturated with methane by stirring with a magnetic stirrer. The hydrates formation was detected by a sudden drop in pressure. There is a small drop in pressure prior to the formation of the hydrate. This drop is due to the dissolution of methane in emulsion prior to the hydrate formation. After hydrate formation, the entire cell was heated at a rate of 1K/hr. During the dissociation process also the same procedure has been followed and the system was kept at each temperature for one hour. When the temperature was increased, the dissociation of hydrates was observed with a substantial increase in pressure. Hydrate dissociation was assumed complete when the heating curve joined the cooling curve. The particle size distribution (PSD) of oil in emulsion was studied using Nano-S90 Zetasizer Ver. 6.34 of Malvern Instruments Ltd., which allows the measurement of droplet diameters in the range of 0.3nm – 10.0 µm. Dynamic Light Scattering is used to measure particle size. This technique measures the diffusion of particles moving under Brownian motion, and converts this to size and a size distribution using the Stokes-Einstein relationship. 5


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Non-Invasive Back Scatter technology (NIBS) is incorporated to give the highest sensitivity simultaneously with the highest dynamic size and concentration range. The particle size distribution curves of three different emulsions are shown in Fig. 2. The sizes of the dispersed oil droplet in emulsion significantly depend on the oil content under similar conditions. It may be seen that the Z-average diameter of dispersed oil droplet increases with increase in oil content in the emulsions. Droplet-size distribution in an emulsion determines, to a certain extent, the stability of the emulsion. The smaller the average sizes of the dispersed water droplets, the tighter the emulsion. Therefore from figure 2 it may be concluded that the emulsions with lower oil content is much stable compare to the others.

3.

RESULTS AND DISCUSSION

3.1 Methane Hydrates Formation in Emulsion. Methane hydrates was formed in the autoclave cell from emulsion prepared with 870 ppm, 1880 ppm and 3560 ppm oil in water. The initial pressure was around 17 MPa for all experiments. A typical behavior of methane hydrates formation and dissociation in presence of emulsion of 870 ppm oil in water is shown in Figure 3. At the beginning of the experiment as the temperature of hydrate cell was decreased, there was a simultaneous small linear decrease in pressure due to gas contraction upon cooling at constant volume. On further reduction of temperature hydrates formation should start at point “A” without metastability (Figure 3). However hydrates began to form at point “B”. Because gas and liquid phase are initially at disordered state on molecular level and as we all know entropy always favors disorder over order, initial hydrates formation is hindered by a long metastable period during which the disorderly gas and liquid water begin to rearrange into the orderly hydrate crystal structure. Hydrates nucleation started at point “B” (Figure 3) is indicated by large pressure drop due to consumption of gas to form hydrates. After formation of hydrates, lowering of temperature stabilizes the hydrates with a simultaneous contraction of unconsumed methane gas.

At point “C” (Figure 3) heating was

started and pressure started to rise, at first slowly. At point “D” (Figure 3) there was a sharp 6


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increase in pressure indicating the beginning of hydrate dissociation. Hydrates dissociation was complete when the heating curve joined the cooling curve at point “A” (Figure 3). It is seen from Table 3 that during hydrates formation, the initial pressure drop (i.e., pressure drop at the time of nucleation) is maximum for 870 ppm oil in water emulsion. This indicates that more methane gas has been encapsulated into hydrates when the percentage of oil in the emulsion is less. It is because as the quantity of oil in emulsion increases less water is available for dissolving methane gas. Nucleation temperature of hydrates formation also decreases with increase in concentration of oil in emulsion for the same reason. From the results it may be concluded that presence of crude oil in water acts as inhibitor for hydrate formation. The result is supported by the other reported study13. 3.2 Methane Hydrates Dissociation in Emulsion. Once the hydrates were formed, the hydrate was heated progressively back to initial conditions. A discontinuous heating resulting in a slow stepwise temperature increase was chosen so that thermodynamic equilibrium in the hydrate, water and methane gas could be achieved at every step. The experimental result shows that with increase in oil contents, the equilibrium curve shifts towards higher pressure and lower temperature (Figure 4). The results are also compared with our earlier work19 for pure water methane system. The dissociation pressure of emulsion-methane system is higher than pure-water methane system at a particular temperature. 3.3 Dissociation Enthalpies of Methane Hydrates in Emulsion. The formation and dissociation of methane hydrates can be represented by the following equation . → +

(1)

Dissociation enthalpies of gas hydrates are usually obtained by direct calorimetric measurement and indirect determination via the Clausius-Clapeyron equation by differentiation of phase equilibrium pressure-temperature data. The Clausius- Clapeyron equation is given by

7


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( )

= −

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(2)

where, p is pressure, T is temperature, z is compressibility factor for gas and is determined by Peng-Robinson equation of state, R is universal gas constant (R=8.314 J.mol-1.K-1) and ∆Hdiss is the molar enthalpy of dissociation of methane gas hydrates. Figure 5 shows the plot between lnP vs 1000/T. As can be seen from Figure 5, ln P vs 1000/T exhibits a very good linear relation. The phase equilibrium pressure increases with the increase in temperature. The dissociation enthalpies of methane hydrates in emulsion were determined via Clausius-Clapeyron equation (Table 4) based on the measured phase equilibrium data. The calculated dissociation enthalpy decreases with the increase in both temperature and oil content in emulsion. The calculated enthalpy of dissociation suggests that when oil content in emulsion increases, the heat required to dissociate hydrates decreases. 3.4 Gas Consumption. A certain time is required to initiate hydrate nucleation when hydrate forming components are placed in the suitable pressure and temperature region. This time lapse is known as induction time 20. Figure 6 shows temperature-pressure response during hydrate formation in emulsion having 870 ppm oil in water under 5.4 ºC subcooling. During hydrate formation there is a sudden drop in pressure and a rise in temperature at the same time. This rise in temperature is due to release of the latent heat of hydrate formation21. In our experiments induction time is measured from the beginning to that time when temperature rise is observed. Table 5 indicates that induction time decreases with increase in oil contents in the emulsion. Figure 7 shows the gas consumption in moles during the experiment. Initially in region 1, gas consumption is very less. Gas consumption increases rapidly after nucleation in region 2 as seen in the Figure 7. The amount of gas consumed during hydrate formation can be calculated from the real gas equation 8


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∆n = n − n = " !

#$

%$ & $

−

#'

%' &'

(

(3)

where, n is the amount of gas consumed when hydrates form, V is gas volume, Pi, Ti and Pf, Tf are the pressure and temperature at initial and final condition respectively, R is the universal gas constant. Compressibility factors (Zi, Zf) are measured, at the corresponding pressure and temperature using Peng-Robinson equation of state. A comparative picture of gas consumption in emulsion is shown in Figure 8. It may be seen from the figure that gas consumption decreases with increase in concentration of oil in emulsion. 3.5 Kinetics of Methane Hydrate Formation. The rates of gas hydrates build up after nucleation, i.e., the hydrate growth stage, is measured in terms of gas consumption rate. As seen in Figure 9, the rate of gas consumption after nucleation increases exponentially. This exponential gas consumption rate can be assumed as a first order reaction. A first order reaction can be defined by the following equation ) = )* ℮,-. ln

0

01

= −23

(4)

(5)

where, N is total number of moles at time t, N0 is initial number of moles, k is rate constant (min-1) and t is time in min. The slope of the curve of ln (N/N0) vs t gives the rate constant (k) of hydrate formation. In Figure 10 we have seen that slope of curve changes with time. This is due to different rate of hydrate formation which gives different reaction rate and hence different rate constant. The results are supported by other reported works22-24. Formation rate i.e. rate by which methane molecules are caged by water molecule in the formation of hydrates is expressed by following expression: 0 .

= −)* 2℮,-.

(6)

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Table 6 shows the methane hydrate formation rate in oil in water emulsion. Initially hydrates formation rate is high, and then it decreases with time. 4.

CONCLUSION The formation and dissociation of methane hydrates in emulsion have been studied. With

increase in oil contents, hydrates nucleation shifted towards lower temperature. The experimental result also indicates that with increase in oil contents, the equilibrium curve shifts towards higher pressure and lower temperature. The calculated dissociation enthalpy decreases with the increase in both temperature and oil content in emulsion. Gas consumption during hydrates formation calculated using real gas equation shows that gas consumption decrease with increase in oil percentage in emulsion. During hydrate growth period gas consumption behavior shows exponential trend. So we use first order kinetics to determine the hydrate formation rate and the result shows kinetic rates in an isochoric cell system are not constant during the whole growth period. ACKNOWLEDGEMENT We gratefully acknowledge the financial assistance provided by University Grants Commission, New Delhi, India, under Special Assistance Program (SAP) to the Department of Petroleum Engineering, Indian School of Mines, Dhanbad, India. REFERENCES 1. Sloan, E. D. Clathrate Hydrate of Natural Gases. 2nd Ed, Marcel Deckker, Inc: New York, 1998. 2. Collett, T.S. Energy resource potential of natural gas hydrates. AAPG Bulletin 2002, 11, 86. 3. Kokal, S. Case Studies of Emulsion Behavior at Resrvoir Conditions. SPE 105534, 15th SPE Middle East Oil and Gas Show and Conference, Bahrain International Exhibition Centre, Kingdom of Bahrain, 11-14 March, 2007. 4. Sloan, E.D. Hydrate Engineering. Society of Petroleum Engineers Inc., Richardson, TX, 2000.

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5. Høiland, S.; Askvik, K.M.; Fotland, P.; Alagic, E.; Barth, T.; Fadnes, F. Wettability of Freon hydrates in crude oil/brine emulsions. Journal of Colloid and Interface Science 2005, 287, 217–225. 6. Høiland, S.; Borgund, A.E.; Barth, T.; Fotland, P.; Askvik, K.M. Wettability of Freon hydrates in crude oil/brine emulsions: the effect of chemical additives. 5th International Conference on Gas Hydrates, Trondheim, Norway, 2005. 7. Greaves, D.; Boxall, J.; Mulligan, J.; Sloan, E.D.; Koh, C.A. Hydrate formation from high water content-crude oil emulsions. Chemical Engineering Science 2008, 63, 4570 – 4579. 8. Talatori, S.; Barth, T. Rate of hydrate formation in crude oil/gas/water emulsions with different water cuts. Journal of Petroleum Science and Engineering 2012, 80, 32–40. 9. Ebeltoft, H.; Yousif, M.; Soergaard, E. Hydrate control during deep water drilling: overview and new drilling fluids. SPE 38567, SPE Annual Technical Conference and Exhibition, San Antonio, Texas, October 5–8, 1997. 10. Power, D.; Slater, K.; Aldea, C.; Lattanzi, S. Gas hydrate inhibited water-based muds for ultra-deepwater drilling: Practical Solutions for Drilling Challenges. Proceedings of AADE National Technology Conference, Houston, Texas, April 1–3, 2003. 11. Fu, B.; Neff, S.; Mathur, A.; Bakeev, K. Novel low dosage hydrate inhibitors for deepwater operations. SPE 71472, SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, September 30–October 3, 2001. 12. Sinquin, A.; Bredzinsky, X.; Beunat, V. Kinetic of Hydrates Formation: Influence of Crude Oils. SPE 71543, SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 30 September–3 October, 2001. 13. Xiang, C.H.; Peng, B.Z.; Liu, H.; Sun, C.Y.; Chen, G.J.; Sun, B.J. Hydrate Formation/Dissociation in (Natural Gas + Water + Diesel Oil) Emulsion Systems. Energies 2013, 6, 1009-1022. 14. Dalmazzone, D.; Kharrat, M.; Lachet, V.; Fouconnier, B.; Clausse, D. DSC and PVT measurements of methane and trichlorofluoromethane hydrate dissociation equilibria in highly concentrated calcium chloride solutions and water-in-oil emulsions. Journal of Thermal Analysis and Calorimetry 2002, 70, 493–505.

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15. Irvin, G.; Li, S.; Simmons, B.; John, V.; Mcpherson, G.; Max, M.; Pellenbarg, R. Control of gas hydrate formation using surfactant systems: Underlying concepts and new applications. Annals of the New York Academy of Sciences 2000, 912, 515–526. 16. Lachance, J.W.; Sloan, E.D.; Koh, C.A. Effect of hydrate formation/dissociation on emulsion stability using DSC and visual techniques. Chemical Engineering Science 2008, 63, 3942–3947. 17. Santander, M.; Rodrigues, R.T.; Rubio, J. Modiﬁed jet ﬂotation in oil (petroleum) emulsion/water separations. Colloids and Surfaces A: Physicochem. Eng. Aspects 2011, 375, 237–244 18. Zouboulis, A.I.; Avranas A. Treatment of Oil-in-Water Emulsions by Coagulation and Dissolved-Air Flotation. Colloids and Surfaces A: Physicochem. Eng. Aspects 2000, 172 153-161. 19. Saw, V.K.; Ahmed, I.; Mandal, A.; Udayabhanu, G.; Laik, S. Methane hydrate formation and dissociation in synthetic seawater. Journal of Natural Gas Chemistry 2013, 21, 625-632. 20. Saw, V.K.; Mandal, A.; Udayabhanu, G.; Laik, S. Methane Hydrate Formation and dissociation in the Presence of Betonite Clay Suspension. Chemical Engineering & Technology 2013, 36(5), 810-818. 21. Karaaslan, U.; Uluneye, E.; Parlaktuna, M. Effect of an anionic surfactant on different type of hydrate structures. Journal of Petroleum Science and Engineering 2002, 35(1-2), 49-57. 22. Turner, D.J; Miller, K. T.; Sloan, E. D. Methane hydrate formation and an inward growing shell model in water in oil dispersions. Chemical Engineering Science 2009, 64, 3996-4004. 23. Saw, V.K.; Gudala M.; Udayabhanu, G.; Mandal, A.; Laik, S. Kinetics of methane hydrate formation and its dissociation in presence of non-ionic surfactant Tergitol. Journal of Unconventional Oil and Gas Resources 2014, 6, 54–59.

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24. Abay, H.K; Svartaas, T.M. On the kinetics of methane hydrate formation: A time-dependent kinetic rate model. Proceedings of the 17th International Conference on gas hydrates (ICGH 2011), Edinburgh, UK, 2011.

Figure 1 . Schematic diagram of Gas Hydrate Setup

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30

Concentration of oil in emulsions 870 ppm 1880 ppm 3560 ppm

Number (%)

20

10

0

0

1000

2000

Oil droplet size (d.nm)

Figure 2: Particle size distributions of different oil-in-water emulsions

12.0

Start Cooling A

11.5

B

11.0

Pressure (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Nucleation

Dissociation Point

10.5

10.0

Growth 9.5

bil Sta

9.0 8.5

C

ion izat

Start Heating 275

280

D (Dissociation Temperature) 285

290

295

Temperature (K)

Figure 3. Pressure-Temperature curve for methane hydrate formation and dissociation in emulsion of 870 ppm oil in water

14


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11.5 11.0

Pressure (MPa)

10.5 10.0 9.5 9.0 19

Pure water, Saw et al. 870 ppm oil in water emulsion 1880 ppm oil in water emulsion 3560 ppm oil in water emulsion

8.5 8.0 286

287

288

289

290

291

292

Temperature (K)

Figure 4. Dissociation Curves for methane hydrates in emulsions

870ppm oil in water emulsion 1880ppm oil in water emulsion 3560ppm oil in water emulsion

2.5 2.45 2.4 lnP (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.35 2.3 2.25 2.2 2.15 3.43

3.44

3.45

3.46

3.47

3.48

3.49

3.5

1000.T -1 (K⁻¹)

Figure 5. Semilogarithimic Plot of pressure vs temperature

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1700

Pressure Temperature

1650 1600

294

292

Hydrate Onset Pressure

Pressure (MPa)

1550 290

Induction Time = ts-t0

1500

288

1450 1400

286

Temperature (K)

1350 284 1300

ts

t0

1250 0

100

282

200

300

400

500

Time (min)

Figure 6. Temperature-Pressure response during hydrate formation in 870 ppm oil in water emulsion under 5.4 ºC subcooling

0.12

1700

1650

Methane Gas Consumed (mole)

0.10

1600 0.08 1550 0.06

Gas Consumption Pressure

1500

Growth Region

0.04

Region 1

1450

Region 3

0.02

Pressure (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

1400

Region 2

0.00 -50

0

50

100

150

200

1350 250

300

350

Time (min)

Figure 7. Gas consumption and reactor pressure vs time during hydrate formation in 870 ppm oil in water emulsion under 5.4 ºC subcooling

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870ppm Oil in Water Emulsion 1880ppm Oil in Water Emulsion 3560ppm Oil in Water Emulsion

Gas Consumption (mole)

0.12

0.10

0.08

0.06

0.04

0.02

0.00 -50

0

50

100

150

200

250

300

350

400

Time (min)

Figure 8. Rate of gas consumption during methane hydrate formation in emulsion

0.76

Hydrates begin to form 0.74

0.72

0.68

Exponential Decrease

Region 3

0.70

Region 1

Free Gas Contents (mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.66

Region 2

0.64 0

50

100

150

200

250

Time (min)

Figure 9. The free gas contents during hydrate growth period for 1880 ppm oil in water emulsion under 7.32° subcooling

17


Energy & Fuels

0.00

K11

ln(N/N0)

-0.01

-0.02

K12

-0.03

K13 -0.04

0

50

100

150

200

250

300

Time (min)

(a) 870 ppm oil in water

0.02 0.00 -0.02

K21

-0.04

ln(N/N0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.06 -0.08

K22

-0.10

K23

-0.12 -0.14 -0.16 0

50

100

150

200

250

Time (min)

(b) 1880 ppm oil in water

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Page 19 of 21

0.00

-0.02

K31 -0.04

ln (N/N0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-0.06

K32

-0.08

K33

-0.10

-0.12 0

50

100

150

200

Time (min)

(c) 3560 ppm oil in water Figure 10: Semi-logarithmic plot of change of moles during hydrate formation

Table 1. Physico-Chemical Characteristics of Crude Oil Sl. No.

Characteristics 3

1

Density (15.5 °C) (kg/m )

855.60

2

Specific Gravity (15.5 °C)

856.10

3

API Gravity (15.5 °C) ( °API)

35.77

4

Viscosity (cp) at 30 °C

5

Acid No.

6

Pour Point ( °C)

525 0.038 mg KOH/g 18

Table 2. SARA distribution of Crude Parameters

Weight Percentage

Saturates

55.6

Aromatics

35.3

Resins

7.9

Asphaltenes

1.2 19


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

Table 3. Nucleation Temperature, Dissociation Temperature, Dissociation Pressure, Dissociation Point and Pressure Drop during methane hydrate in different emulsions Emulsion Z-avg. dia. of Nucleation (Oil in oil droplet Temperature Water) (K)

Dissociation Temperature (K)

Dissociation Dissociation Pressure Point(K) (MPa)

Pressure Drop (MPa)

870 ppm 1880 ppm 3560 ppm

286.57 286.37 285.86

11.35 11.20 11.23

1.63 0.63 0.57

466 nm 712 nm 955 nm

285.17 283.96 281.86

291.12 290.08 289.60

Table 4. Calculated dissociation enthalpy of methane hydrates in emulsion Emulsion 870ppm

1880ppm

3560ppm

T (K) 287.09 287.95 288.80 289.64 290.26 291.12 287.37 288.26 289.17 290.08 286.70 287.64 288.60 289.60

P (MPa) 9.02 9.58 10.13 10.65 11.12 11.35 10.10 10.47 10.91 11.20 10.12 10.45 10.87 11.23

Z-factor ∆Hdiss (kJmol-1) 0.8158 33.50 0.8106 33.29 0.8063 33.11 0.8030 32.97 0.8000 32.85 0.8003 32.86 0.8029 21.72 0.8012 21.68 0.7991 21.62 0.7988 21.61 0.8010 20.24 0.7997 20.21 0.7980 20.16 0.7973 20.15

Table 5. Induction Time for methane hydrates formation in emulsion Emulsion 870ppm 1880ppm 3560ppm

Induction Time (min) 160 103 62

Subcooling (ºC) 5.40 7.32 7.63

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 6. Methane hydrates formation rate in emulsion Emulsion 870ppm

1880ppm

3560ppm

Rate Constant (min-1) K11 8.61×10-4 K12 1.81×10-4 K13 7.27×10-5 K21 3.27×10-3 K22 8.16×10-4 K23 2.51×10-4 K31 2.61×10-3 K32 4.20×10-4 K33 2.31×10-4

Formation rate (molmin-1) 6.35×10-4 1.32×10-4 5.23×10-5 2.34×10-3 5.57×10-4 1.65×10-4 2.03×10-3 3.14×10-4 1.69×10-4

21


Methane Hydrate Formation and Dissociation in Oil-in-Water Emulsion

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